1. Field of the Invention
The present invention relates to a magnetic-balance-system current sensor using a magnetoresistive element (a tunneling magnetoresistive element (TMR) element or a giant magnetoresistive element (GMR) element).
2. Description of the Related Art
Motors are driven using electricity generated by engines in electric vehicles. The magnitude of a current used to drive such a motor is detected by, for example, a current sensor. Such a current sensor is a sensor in which a magnetic core, a portion of which has a notch (a core gap), is disposed around a conductor, and in which a magnetic detector element is disposed in the core gap. Examples of such a current sensor include a magnetic-balance-system current sensor illustrated in FIG. 13 (Japanese Unexamined Patent Application Publication No. 8-15322).
The magnetic-balance-system current sensor illustrated in FIG. 13 has a feedback coil 2 that is wound around a magnetic core 1 in such a direction as to cancel out a magnetic field B1 generated by a measurement target current I. In the magnetic-balance-system current sensor, when the measurement target current I flows, an output voltage is generated in a magnetic detecting element 3 by the magnetic field B1 generated by the current. A voltage signal that is output from the magnetic detecting element 3 is converted into a current. The current is fed back to the feedback coil 2. The magnetic-balance-system current sensor operates so that a magnetic field (a cancelling magnetic field) B2, which is generated by the feedback coil 2, and the magnetic field B1, which is generated by the measurement target current I, cancel each other out and a magnetic field is always zero. In this case, a feedback current flowing through the feedback coil is converted into a voltage, and the voltage is obtained as an output. In FIG. 13, reference numeral 4 denotes an amplifier, and reference numeral R denotes a detection resistor.
In recent years, values of currents that are handled have been rising along with higher output and higher performance of electric vehicles. For this reason, magnetic saturation needs to be prevented when a large current flows. In order to prevent magnetic saturation, the size of magnetic cores needs to be increased. However, there is a problem that, when the size of magnetic cores is increased, the size of current sensors is also increased. Furthermore, in association with the higher performance of electric vehicles, currents need to be more accurately measured, and there is a demand for high-sensitivity current sensors.